1. Field of the Invention
The invention relates to a method of securing an implantable medical device onto a balloon. Particularly, the invention relates to a method of securing an implantable medial device by coating the outer surface of the balloon with a film-forming polymer and subsequently securing the implantable medical device to the surface of the balloon.
2. Description of Related Art
Cardiovascular disease is prevalent in the United States and in other parts of the world. One manifestation of cardiovascular disease is atherosclerosis, which is the buildup of plaque (or fatty deposits) on the walls of blood vessels, such as coronary arteries. This buildup of plaque can grow large enough to reduce blood flow through the blood vessel. Serious damage results when an area of plaque ruptures and forms a clot, which travels to another part of the body. If the blood vessels that feed the heart are blocked, a heart attack results. If the blood vessels to the brain are blocked, a stroke results. Thus, atherosclerosis can be fatal for some people.
Typically, atherosclerosis is treated by percutaneous transluminal coronary angioplasty (PTCA). This procedure generally entails introducing a catheter assembly into the cardiovascular system of a patient via the brachial or femoral artery, and advancing the catheter assembly through the coronary vasculature until a balloon portion thereon is positioned across an occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against the atherosclerotic plaque of the lesion to remodel the vessel wall. Subsequently, the balloon is deflated to allow the catheter assembly to be withdrawn from the vasculature.
While PTCA is widely used, it suffers from two unique problems. First, the blood vessel may suffer acute occlusion immediately after or within the initial hours after the dilation procedure. Second, the re-narrowing of an artery or other blood vessel after an initially successful angioplasty sometimes results. This blood vessel re-narrowing is commonly referred to as “restenosis,” which typically occurs within the first six months after angioplasty. Restenosis is believed to be due to, among other things, the proliferation and migration of cellular components from the arterial wall, as well as through geometric changes in the arterial wall referred to as “remodeling.”
To reduce the restenosis rate of angioplasty alone and to strengthen the dilated area, physicians typically implant a tubular endoprosthesis, generally called a stent, inside the vasculature at the site of the lesion or blocked segment. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel or to maintain its patency. A typical stent-delivery system for balloon expandable stents is characterized by a catheter equipped with a dilation balloon and a stent mounted on the balloon, otherwise known as a stent delivery system. In such a system, the stent is slipped over a folded catheter balloon and crimped in place. Additionally, the stent or implantable medical device may be loaded with one or more therapeutic agents, such as antiproliferatives, for delivery to the target lesion. The stent delivery device enters the vasculature of a patient and travels through a tortuous path to the site of the lesion. The physician positions the stent across the lesion and deploys the stent so that the stent forces the plaque against the inside wall of the blood vessel (or lumen) and maintains its expanded configuration so that the patency of the blood vessel is maintained.
A fairly common problem with stent deployment, however, is slippage and early unintentional release of the stent. The stent may pop off the balloon during inflation, or slip backward off the balloon during steering to the intended site of release. This condition most often occurs in small or heavily occluded arteries where contact with either the arterial wall or the lesion to be treated forces the stent off of the catheter balloon. Additionally, passage of the exposed stent through any valves may cause the stent to be dislodged from the balloon. If the stent is dislodged from or moved relative to the balloon, the system will not correctly implant the stent into body lumen. Alternatively, if a stent slips off of the balloon, or if it slips forward off the balloon in a partially expanded condition, the steps necessary to remove the stent can severely damage an artery, and may even require grossly invasive surgery to remove the stent.
Different methods have been attempted to maintain the position of the stent on the expandable member. One such method involves a protective sheath surrounding the catheter and stent assembly, which is retracted prior to inflation of the expandable member. The use of the sheath, however, increases the profile of the catheter assembly which must traverse narrow vessels. Other methods include applying dissolvable bands to the stent surface to hold the stent in place. The bands, however, also add significantly to the outer diameter of the stent assembly and leave exposed sharp and irregular contours of the stent assembly.
Other methods to increase stent retention include providing protrusions on the balloon or on the catheter near the balloon, the protrusions having shoulders above and/or below the stent location which bears against the stent when it is subjected to an axial force. Alternatively, protrusions may be provided on the external surface of the balloon, or on a sleeve located between the stent and the balloon, which protrude into the holes through the wall of the stent. However, the procedures used to make such balloons tend to lead to weakening of the balloon wall, an increase in the pressure required to inflate the balloon, and/or require additional manufacturing steps and steps for careful positioning of the stent upon the balloon.
Other methods include coating the exterior surface of the stent delivery device with a film-forming polymer coating, which includes a solvent. However, the solvent that is present in the coating tends to remove or redistribute any drugs that have been loaded on the stent.
Accordingly, it would be beneficial to provide a catheter balloon having improved stent retention without inhibiting balloon or catheter function. Particularly, the stent mounting procedure must not damage a drug or drug delivery matrix on the stent delivery system.